The object of these investigations is to identify some of the major events of early embryogenesis in Drosophila melanogaster. Two major lines of attack are being pursued: (1) A program to isolate temperature sensitive mutants which disrupt normal embryogenesis is in progress. These mutants will be examined in detail and data obtained on: (a) which periods of development are sensitive to temperature shifts, (b) the mutations' location on the genetic map, (c) histologically observable developmental defects of lethal embryos and (d) possible alterations in macromolecular synthesis during development. These studies will provide us with some broad outlines of the range, type, and number of events which are of importance in the regulation of early development. (2) The role of a known gene, rudimentary, in controlling development will be examined at both the genetic and biochemical levels. A hypothesis which attempts to explain the pleiotropic effects of this mutant at the biochemical level is presented and a number of experimental tests of its validity proposed. BIBLIOGRAPHIC REFERENCES: Fausto-Sterling, A., A. Weiner and M. Digan (1977) "Analysis of a newly-isolated temperature sensitive maternal effect mutation in D. melanogaster." J. Exp. Zool., in press, Fausto-Sterling, A. (1977) Studies on the female sterile mutant fused of Drosophila melanogaster. II. An analysis of aspartate transcarbamylase and dehydroorotase activities in wild type and rudimentary strains." Biochemical Genetics, in press.